Method of atomic layer etching of oxide

ABSTRACT

In one exemplary embodiment, described herein is an ALE process for etching an oxide. In one embodiment, the oxide is silicon oxide. The ALE modification step includes the use of a carbon tetrafluoride (CF4) based plasma. This modification step preferentially removes oxygen from the surface of the silicon oxide, providing a silicon rich surface. The ALE removal step includes the use of a hydrogen (H2) based plasma. This removal step removes the silicon enriched monolayer formed in the modification step. The silicon oxide etch ALE process utilizing CF4 and H2 steps may be utilized in a wide range of substrate process steps. For example, the ALE process may be utilized for, but is not limited to, self-aligned contact etch steps, silicon fin reveal steps, oxide mandrel pull steps, oxide spacer trim, and oxide liner etch.

This application is a continuation of U.S. patent application Ser. No.16/401,441, entitled “Method of Atomic Layer Etching of Oxide,” filedMay 2, 2019, which claims priority to U.S. Provisional PatentApplication No. 62/670,459, entitled, “Method of Atomic Layer Etching ofOxide,” filed May 11, 2018 and U.S. Provisional Patent Application No.62/684,878, entitled, “Method of Atomic Layer Etching of Oxide,” filedJun. 14, 2018 the disclosure of which is expressly incorporated herein,in its entirety, by reference.

BACKGROUND

The present disclosure relates to the processing of substrates in plasmaprocess equipment. In particular, it provides a method to control plasmaetching of layers comprising oxides.

The use of plasma systems for the processing of substrates has long beenknown. For example, plasma processing of semiconductor wafers is wellknown. One well known use of plasma processing is for etching ofsubstrates. Plasma etching presents numerous technical challenges.Further as geometries for structures and layers on substrates continueto shrink, tradeoffs between etch selectivity, profile, aspect ratiodependent etching, uniformity, etc. become more difficult to manage. Inorder to achieve desired process performance, variable settings of theplasma processing equipment can be adjusted to change the plasmaproperties. These settings include, but are not limited to gas flowrates, gas pressure, electrical power for the plasma excitation, biasvoltages, etc., all as is known in the art. However, as geometriescontinue to shrink it has been found that sufficient control of ionenergy, ion flux, radical flux, etc. that results from the settings ofthe plasma processing equipment is not satisfactory to achieve thedesired etch results.

One technique to improve plasma etching has been to utilize atomic layeretch (ALE) plasma processes. ALE processes are general known to involveprocesses which remove thin layers sequentially through one or moreself-limiting reactions. Thus, ALE processes offer improved performanceby decoupling the etch process into sequential steps of surfacemodification and removal of the modified surface, thereby allowing thesegregation of the roles of radical flux and ion flux and energy. Suchprocesses often include multiple cyclic series of layer modification andetch steps. The modification step may modify the exposed surfaces andthe etch step may selectively remove the modified layer. Thus, a seriesof self-limiting reactions may occur. As used herein, an ALE process mayalso include quasi-ALE processes. In such processes, a series ofmodification and etch step cycles may still be used, however, theremoval step may not be purely self-limiting as after removal of themodified layer, the etch substantially slows down, though it may notcompletely stop. In either case, the ALE based processes include acyclic series of modification and etch steps.

It would be desirable to provide an improved ALE process. Morespecifically, it would be desirable to provide an improved ALE processfor etching of oxides.

SUMMARY

In one exemplary embodiment, described herein is an ALE process foretching oxides. In one embodiment, an ALE process for etching siliconoxide is provided. However, it will be recognized that the conceptsdescribed herein may be applicable to the etching of other oxides, forexample, metal oxides, germanium dioxide, silicon oxynitride, etc. In anembodiment, an ALE modification step includes the use of a fluorinatedhydrocarbon such as a carbon tetrafluoride (CF4) based plasma, whereinthe fluorinated hydrocarbon can be perfluorinated hydrocarbon and isgaseous at the working temperatures of the methods described herein.This modification step preferentially removes oxygen from the surface ofthe silicon oxide, providing a modified surface layer, which can be asilicon rich surface and can be a monolayer. The ALE removal stepincludes the use of a hydrogen (H2) based plasma. This removal stepremoves the silicon enriched layer formed in the modification step. Thesilicon oxide etch ALE process utilizing CF4 and H2 steps may beutilized in a wide range of substrate process steps. For example, theALE process may be utilized for, but is not limited to, self-alignedcontact etch steps, silicon fin reveal steps, oxide mandrel pull steps,oxide spacer trim, and oxide liner etch.

In one embodiment, a method for etching a substrate is provided. Themethod may comprise providing a first layer comprising silicon oxide,the first layer to be etched selective to a second layer. The methodfurther comprises exposing the first layer to a first plasma comprisingCF4 to modify at least a surface of the first layer to form a modifiedsurface layer, the modified surface layer being silicon rich compared tothe remainder of the first layer. The method further comprises exposingthe modified surface layer to a second plasma comprising H2, the plasmacomprising H2 removing at least a portion of the modified surface layer.A combination of use of the first plasma and the second plasma reducesat least a portion of a thickness of the first layer.

In another embodiment, a method for etching a substrate is provided. Themethod comprises providing a first layer comprising silicon oxide. Themethod further comprises performing an atomic layer etch process to etchthe first layer, the atomic layer etch process comprising multiplecycles of (1) a surface modification step comprising a first plasma, thefirst plasma comprising CF4 and (2) a removal step following the surfacemodification step, the removal step comprising a second plasma, thesecond plasma comprising H2.

In another embodiment, a method for reducing a thickness of a siliconoxide layer on a substrate is provided. The method comprises at leastone cycle of (a) exposing the silicon oxide layer to a perfluorinatedhydrocarbon plasma to modify the surface of the silicon oxide layerfollowed by (b) exposing a product of step (a) to an elemental hydrogenplasma to remove at least a portion of the surface modified in step (a).

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present inventions and advantagesthereof may be acquired by referring to the following description takenin conjunction with the accompanying drawings, in which like referencenumbers indicate like features. It is to be noted, however, that theaccompanying drawings illustrate only exemplary embodiments of thedisclosed concepts and are therefore not to be considered limiting ofthe scope, for the disclosed concepts may admit to other equallyeffective embodiments.

FIG. 1 illustrates one exemplary process flow utilizing the etch methodsdescribed herein.

FIGS. 2A-2C illustrate the surface mechanisms which may occur in thesteps of the methods of one embodiment described herein.

FIG. 3 illustrates one exemplary table contrasting the amount of siliconoxide etched by carbon tetrafluoride plasma alone, hydrogen plasmaalone, and carbon tetrafluoride plasma followed by hydrogen plasma.

FIG. 4 illustrates a graph of silicon oxide removed by the hydrogenplasma after the carbon tetrafluoride plasma.

FIG. 5 illustrates a graph comparing the total silicon oxide etched andthe amount of silicon oxide etched per cycle as a function of the numberof first and second plasma cycles described herein.

FIG. 6 illustrates a graph showing exemplary amounts of silicon oxide,silicon nitride, and polysilicon using the methods described herein.

FIGS. 7A-7C illustrate a representative application of the methoddescribed herein in a self-aligned contact application.

FIGS. 8A-8B illustrate a representative application of the method hereinin a fin reveal application.

FIGS. 9A-9B illustrate a representative application of the methoddescribed herein in an oxide mandrel pull application.

FIGS. 10A-10B illustrate a representative application of the methoddescribed herein in a silicon oxide spacer trim application.

FIGS. 11A-11B illustrates a representative application of the methoddescribed herein in a silicon oxide liner etch application.

FIGS. 12-14 illustrate a representative flow diagrams of the methodsdisclosed herein.

DETAILED DESCRIPTION

In one exemplary embodiment, described herein is an ALE process foretching oxide. In one embodiment, an ALE process for etching siliconoxide is provided. However, it will be recognized that the conceptsdescribed herein may be applicable to the etching of other oxides. Forexample, metal oxides where formation of volatile metal hydride andsilicon oxynitride may be applicable. The ALE modification step includesthe use of a fluorinated hydrocarbon plasma such as a carbontetrafluoride (CF4) based plasma. However, it will be recognized withthe benefit of this disclosure that other fluorocarbon gases may beutilized singularly or in combination with CF4 to achieve themodification step. For example, other fluorocarbons may include, but arenot limited to, hexafluorobutadiene (C4F6) and octafluorocyclobutane(C4F8). This modification step preferentially removes oxygen from thesurface of the silicon oxide, providing a silicon rich layer on thesurface of the substrate. The ALE removal step includes the use of ahydrogen (H2) based plasma. This removal step removes the siliconenriched layer formed in the modification step. The silicon enrichedlayer can be a monolayer. The silicon oxide etch ALE process utilizingCF4 and H2 steps may be utilized in a wide range of substrate processsteps. For example, the ALE process may be utilized for, but is notlimited to, self-aligned contact etch steps, silicon fin reveal steps,oxide mandrel pull steps, oxide spacer trim steps, and oxide liner etchsteps.

More specifically, FIG. 1 illustrates an exemplary ALE process foretching oxide according to the techniques disclosed herein. In FIG. 1 ,the process 100 is illustrated by the initial delivery 110 of asubstrate into a plasma processing region. Next a carbon tetrafluorideplasma is ignited in step 1, block 120. The substrate is then subjectedto step 2, block 130 where a hydrogen plasma is ignited and thesubstrate exposed to the hydrogen plasma. It should be noted that argonor other inert gas can be used as a co-feed with the carbontetrafluoride and hydrogen. If additional etching is desired, thesubstrate is returned to step 1 block 120 via line 125 for an additionalcycle of steps 1 and 2. If etching is complete, the substrate is removedfrom the plasma processing region as shown in removal block 140.

More specifically, as shown in FIG. 1 , the ALE process starts with aCF4/Argon plasma step 1 block 120 to operate as a layer modificationstep. Then a H2/Argon plasma step is performed as shown in step 2 block130 to remove the modified layer generated in the layer modificationstep. The modification and removal steps may then be repeated asufficient number of cycles so as to complete the removal of the desiredamount of oxide. In one embodiment, the oxide is silicon oxide.

FIGS. 2A-2C illustrate the exemplary mechanisms that occur in each stepof the ALE process of FIG. 1 . It will be recognized that the mechanismsdisclosed are merely exemplary, and other mechanisms may occur. FIGS.2A-2C are illustrative and not intended to show precise substratemodifications. As shown in FIG. 2A, substrate 210 includes silicon atoms211 and oxygen atoms 212. As shown in FIG. 2A an upper oxygen layer 213is provided. After being exposed to a carbon tetrafluoride plasma, thesubstrate 210 is modified to form a silicon-enriched layer 225 on thesubstrate 220, which includes a oxygen depleted zone 213A, as shown inFIG. 2B. Notably, the upper oxygen layer 213 of FIG. 2A has been atleast partially reduced resulting in a silicon-enriched layer 225. Thesilicon-enriched layer 225 of substrate 220 is then subjected to ahydrogen plasma, resulting in a final substrate 230 as shown in FIG. 2C.As shown in FIG. 2C, the removal of a silicon-enriched layer 225 isselective to silicon oxide.

Thus, as shown in FIGS. 2A-2C, regions at the surface of the siliconoxide become silicon rich as oxygen is removed from the silicon oxidesurface in the modification step 215. Then, in the removal step (step2), shown in FIG. 2C exposure to hydrogen plasma results in the siliconbeing removed due to the etching action of the hydrogen plasma, whichcan be a in one exemplary embodiment, a H2/Argon plasma. This processmay be repeated in multiple cycles to incrementally remove oxygen andthen remove the silicon rich layer down through the silicon oxide layeruntil the preferred amount of silicon oxide removal is achieved.

The impact of using a two-step ALE process versus merely using one orthe other steps is shown in FIG. 3 . As shown in the graph of FIG. 3 ,the amount of oxide etched by just step 1 and step 2 alone, as depictedby step 1, bar 310 and step 2, bar 330 is contrasted to the use of bothsteps 1 and 2 in combination as described above, as depicted in steps 1and 2 combination, bar 320. The Y axis is denoted as oxide etched(Angstroms).

The self-limiting effect of the two-step ALE process is shown in FIG. 4. FIG. 4 illustrates the amount of oxide that is removed by the secondstep (hydrogen plasma) after performance of the first step (carbontetrafluoride plasma). As shown in the line 410 of the graph of FIG. 4 ,for increasing etch times of the second step, the amount of siliconoxide removed from the surface relatively saturates over time. In thegraph, the Y axis is denoted as oxide EA (Angstroms) with the X axisshown as hydrogen plasma time in seconds.

FIG. 5 illustrates the total oxide etched and the amount of oxide etchedper cycle as a function of the number of cycles of the ALE steps. Thegraph in FIG. 5 depicts oxide etched (Angstroms) in the left Y axis andetch per cycle (Angstroms per cycle) in the right Y axis. The X axisshows the number of cycles of steps 1 and 2. The oxide etched is denotedas line 520 and the etch per cycle is denoted as line 510.

FIG. 6 illustrates exemplary amounts of silicon oxide, silicon nitrideand polysilicon for an ALE process as described herein per 120 secondsof the second step (H2/Argon plasma) as a function of the pressure ofthe second step. As can be seen in the graph in FIG. 6 , the two-stepALE process may provide a highly selective process for etching siliconoxide or silicon nitride or polysilicon. It will be recognized that theetch amounts, etch rates, materials, and so on of FIGS. 2-6 are merelyexemplary and that the concepts described herein may be used with otherALE processes having other characteristics and qualities. In the graphof FIG. 6 , line 610 denotes the amount of both oxide and siliconnitride etched (as the graph lines for each material substantiallyoverlap) and line 620 denotes the polysilicon etched.

The two-step ALE process described herein may be utilized in a widevariety of applications at various points of differing substrate processflows. For example, the ALE process may be used at self-aligned contactetch steps, silicon fin reveal steps, oxide mandrel pull steps, oxidespacer trim steps, and oxide liner etch steps. FIGS. 7A-11B provideexemplary uses of the ALE process described herein in a variety ofsubstrate process flows. It will be recognized that the ALE processdescribed herein may be utilized in many other substrate processingapplications. For example, a variety of process steps in which selectiveremoval of silicon oxide may be desired may suitably utilize thetechniques disclosed herein. In one embodiment, the techniques may beutilized in semiconductor substrate processing, and more particular,semiconductor wafer processing.

FIGS. 7A-7C illustrate an application of the ALE process techniquesdisclosed herein in a self-aligned contact application. As shown in FIG.7A, a plurality of layers are formed on a substrate 705. Substrate 705may be any substrate for which the use of patterned features isdesirable. For example, in one embodiment, substrate 705 may be asemiconductor substrate having one or more semiconductor processinglayers formed thereon. In one embodiment, the substrate 705 may be asubstrate that has been subject to multiple semiconductor processingsteps which yield a wide variety of structures and layers, all of whichare known in the substrate processing art and may be considered to bepart of the substrate 705. In the exemplary embodiment of FIG. 7A, anoxide layer 710, may be provided under an amorphous silicon layer 715. Asilicon nitride hard mask 720 may be provided along with a siliconnitride spacer 725 as shown. An oxide layer 730 may be formed over andbetween the structures formed by the amorphous silicon layer 715 asshown. An organic dielectric layer 735 may be provided over which asilicon anti-reflective coating 735 is provided. A patterned photoresistlayer 745 is also provided.

As shown in FIG. 7A, the oxide layer 730 is formed in and over a regionwhere contacts may ultimately be desired to be formed. FIG. 7Billustrates removal of the various layers (which may be performed viaconventional process steps) to a point where the oxide layer 730 ispartially etched. In one example, a traditional oxide fluorocarbon etchmay be utilized to partially etch oxide layer 730 to achieve thestructure shown in FIG. 7B having a remaining portion 730A of the oxidelayer 730. Then the remaining portion 730A may be removed such as shownin FIG. 7C. As shown in FIG. 7C, the remaining portion 730A has beenremoved by utilizing the highly selective two step ALE process describedherein, for example a fluorocarbon plasma etch chemistry such as carbontetrafluoride followed by a second step such as a hydrogen plasma step.Thus, a process is provided to remove the remainder of the oxide in thecontact region in a highly selective manner to the underlying siliconnitride spacer layer to achieve. Thus, post etch a structure havingcontact regions 750 may be obtained.

FIGS. 8A-8B illustrate an application of the ALE process techniquesdisclosed herein in a fin reveal application. As shown in FIG. 8A, a fin807 on a substrate is protected by a silicon nitride layer 805. Siliconoxide 803 is provided around the fin 807 regions as shown in FIG. 8A.The silicon oxide 803 may be removed (in this example partially removed)in this application via use of the ALE process disclosed herein. In thismanner, the silicon oxide 803 may be removed selectively to the siliconnitride layer 805 to achieve a structure such as shown in FIG. 8B.

FIGS. 9A-9B illustrates an application of the ALE process techniquesdisclosed herein in an oxide mandrel pull application. As shown in thefigures, a silicon oxide mandrel 910 may be surrounded by silicon orsilicon nitride layer 912, such as for example, spacers formed on thesides of the silicon oxide mandrel 910. The ALE process described hereinmay be utilized to remove (pull) the silicon oxide mandrel 910 from thesubstrate, leaving the spaces 931 remaining post-etch in FIG. 9B.

FIGS. 10A-10B illustrate an application of the ALE process techniquesdisclosed herein in a silicon oxide spacer trim application. As shown inthe FIG. 10A, a silicon oxide spacer 1010A may be formed around astructure 1012 (for example a silicon or silicon nitride structure). Thesilicon oxide spacer 1010 may also be provided over an etch stop layer1015. The ALE process described herein may be utilized to provide aspacer trimming step to trim a portion of the silicon oxide spacer 1010Ain a controlled manner so as to narrow the silicon oxide spacer 1010Awidth to produce a narrower silicon oxide spacer 1010B as shown in theFIG. 10B.

FIG. 11 illustrates an application of the ALE process techniquesdisclosed herein in a silicon oxide liner etch application. As shown inthe figure, a silicon oxide liner 1110 may line the sides of a structure1112 (for example a silicon or silicon nitride structure) as shown inFIG. 11A. The silicon oxide liner 1110 may then be removed via oxideliner etch 1122 in a manner selective to the structure 1112 to produce astructure as shown in the FIG. 11B by utilizing the ALE processdisclosed herein as an oxide liner etch.

It will be recognized that the process flows described above are merelyexemplary, and many other processes and applications may advantageouslyutilize the techniques disclosed herein. FIGS. 12-14 illustrateexemplary methods for use of the processing techniques described herein.It will be recognized that the embodiments of FIGS. 12-14 are merelyexemplary and additional methods may utilize the techniques describedherein. Further, additional processing steps may be added to the methodsshown in the FIGS. 12-14 as the steps described are not intended to beexclusive. Moreover, the order of the steps is not limited to the ordershown in the figures as different orders may occur and/or various stepsmay be performed in combination or at the same time.

In FIG. 12 , a method of etching a substrate is shown. The methodincludes step 1205 of providing a first layer comprising silicon oxide,the first layer to be etched selective to a second layer. The methodfurther includes step 1210 of exposing the first layer to a first plasmacomprising CF4 to modify at least a surface of the first layer to form amodified surface layer, the modified surface layer being silicon richcompared to the remainder of the first layer. The method furtherincludes step 1215 of exposing the modified surface layer to a secondplasma comprising H2, the plasma comprising H2 removing at least aportion of the modified surface layer. In the method, a combination ofuse of the first plasma and the second plasma reduces at least a portionof a thickness of the first layer.

In FIG. 13 , a method for etching a substrate is shown. The methodincludes step 1305 of providing a first layer comprising silicon oxide.The method further includes step 1310 of performing an atomic layer etchprocess to etch the first layer. In the method, the atomic layer etchprocess may include a step of multiple cycles of (1) a surfacemodification step comprising a first plasma, the first plasma comprisingCF4 and (2) a removal step following the surface modification step, theremoval step comprising a second plasma, the second plasma comprisingH2.

In FIG. 14 , a method for reducing a thickness of a silicon oxide layeron a substrate is shown. The method includes step 1405 of performing atleast one cycle of (a) exposing the silicon oxide layer to aperfluorinated hydrocarbon plasma to modify a surface of the siliconoxide layer. Step 1405 is followed by step 1410 of (b) exposing aproduct of step (a) to an elemental hydrogen plasma to remove at least aportion of the surface modified in step (a).

Further modifications and alternative embodiments of the inventions willbe apparent to those skilled in the art in view of this description.Accordingly, this description is to be construed as illustrative onlyand is for the purpose of teaching those skilled in the art the mannerof carrying out the inventions. It is to be understood that the formsand method of the inventions herein shown and described are to be takenas presently preferred embodiments. Equivalent techniques may besubstituted for those illustrated and described herein and certainfeatures of the inventions may be utilized independently of the use ofother features, all as would be apparent to one skilled in the art afterhaving the benefit of this description of the inventions.

What is claimed is:
 1. A method for etching a substrate, comprising:receiving the substrate into a plasma processing chamber, the substratecomprising a silicon-containing film; exposing the substrate to a firstplasma comprising carbon tetrafluoride (CF4) to form a silicon enrichedlayer on the silicon-containing film; and exposing the substrate to asecond plasma comprising hydrogen (H2) to remove the silicon enrichedlayer on the silicon-containing film, wherein a combination of use ofthe first plasma and the second plasma reduces at least a portion of athickness of the silicon-containing film.
 2. The method of claim 1,wherein the first plasma modifies at least a surface of the substrate toform a modified surface layer, the modified surface layer being siliconrich compared to a remainder of the substrate, and wherein the secondplasma removes at least a portion of the modified surface layer.
 3. Themethod of claim 1, wherein the method for etching the substrate is partof a self-aligned contact application.
 4. The method of claim 1, whereinthe method for etching the substrate is part of a fin revealapplication.
 5. The method of claim 1, wherein the method for etchingthe substrate is part of an oxide mandrel pull application.
 6. Themethod of claim 1, wherein the method for etching the substrate is partof a silicon oxide spacer trim application.
 7. The method of claim 1,wherein the method for etching the substrate is part of a silicon oxideliner etch application.
 8. The method of claim 1, wherein the siliconcontaining film comprises silicon oxide, silicon oxynitride, and/orpolysilicon.
 9. A method for etching a substrate, comprising: receivingthe substrate into a plasma processing chamber, the substrate comprisinga film layer on a surface of the substrate; exposing thesilicon-containing film layer to a first plasma comprising carbontetrafluoride (CF4) to form a metal hydride layer on the film layer or asilicon enriched layer on the film layer; and exposing thesilicon-containing film layer to a second plasma comprising hydrogen(H2) to remove the metal hydride layer or the silicon enriched layer,alternating between the exposing the first plasma and the second plasmato the substrate to reduce at least a portion of a thickness of thesubstrate.